Nonvolatile memory is used by electronic equipment to store data. Data stored within nonvolatile memory is retained even when power to the electronic equipment is cut off. Therefore, nonvolatile memories are typically used in applications in which the user requires that data survive power interruptions to the electronic equipment or power interruptions to the memory itself, such as during physical transfer of a memory device from one piece of computer equipment to another. For example, it is been found useful to store data necessary to boot up a computer system in a nonvolatile memory inside the computer so that the data will always be available to the computer each time a user turns it on. For another example, a standard memory card contains a nonvolatile memory that allows a user to store data on the memory card at a first computer and then access the data using a second computer into which the memory card is subsequently inserted.
There are many types of nonvolatile memory storage devices, the most popular of which is the electrically programmable read only memory (EPROM). Another type of nonvolatile memory is the electrically programmable and electrically erasable read only memory (EEPROM) that was developed to erase and to rewrite the data contained in the memory on a byte-by-byte basis. More recently, a new category of nonvolatile memories has emerged known as flash EEPROMs. In a flash memory, an entire array of data, called a block of data, is simultaneously erased. Although a flash memory is capable of storing relatively large amounts of data in comparison to other nonvolatile memories, flash also exhibits several disadvantages as well.
For example, flash memories have been found to be vulnerable to inadvertent write and erase operations. During a write operation, a flash memory is programmed by storing the desired data in the device. The flash memory is erased in blocks. Once data has been stored in a flash memory by a series of write operations, the data may be read from the flash memory any number of times without incident. Software bugs or computer glitches have been known to inadvertently corrupt data contained within a flash memory, however, by accidentally causing computer data to be written over or erased. In addition, when a computer system to which a flash memory is coupled is turned on, off, or is reset, the supply voltage to the memory fluctuates. For example, a flash device may be inadvertently placed in a write or erase mode, particularly during power-up or power-down, when system control signals are indeterminate, making the flash device susceptible to data corruption.
To protect the data stored within a nonvolatile memory from these and other modes of corruption, protection circuitry is designed into the memory. This protection circuitry, shuts off or "locks out" access to the memory during periods of vulnerability. For example, one type of protection circuit is coupled to particular control pins of the memory such that when a particular signal generated by the computer system is registered by these pins, the protection circuit inside the memory places one or more blocks into lockout mode, thereby preventing all write and erase operations to these memory blocks. By preventing all write and erase operations during lockout, inadvertent write and erase operations, that would corrupt the integrity of the data, are avoided. Another type of protection circuit contained within a memory is coupled to one or more voltage supply input pins of the device. This type of protection circuit measures the supply voltage to the memory and places one or more memory blocks of the device into lockout mode when the supply voltage fluctuates during power-up and power-down of the computer system. In doing so, data corruption and device destruction is avoided.
Unfortunately, these various protection circuits can occupy a substantial amount of space within a memory, space that could otherwise be used to store data. In addition, the extra pins associated with various protection circuits increase the overall size and complexity of the memory. As a result, protection circuits and their associated control pins increase the cost and reduce the storage capacity of nonvolatile memories.